1. Field of the Invention
The present invention relates to a color display tube. It relates in particular to a color display tube characterized by electrodes that constitute a main lens for focusing three electron beams on a phosphor screen.
2. Description of Related Art
A color display tube device generally has an envelope including a panel and a funnel that is connected integrally therewith. An electron gun is disposed in a neck portion of the funnel. A deflecting device is mounted outside the funnel. A phosphor screen is formed on an inner surface of the panel so as to face a shadow mask. Three electron beams emitted from the electron gun are deflected by horizontal and vertical deflection magnetic fields generated by the deflecting device and excite the phosphor screen while scanning it horizontally and vertically, thereby displaying a color image.
The magnetic fields generated by the deflecting device used in such a color display tube device generally has a self-convergence structure, in which the three electron beams are converged on a screen. For this purpose, the horizontal deflection magnetic field and the vertical deflection magnetic field are distorted to have a pincushion shape and a barrel shape respectively. Accordingly, the three electron beams passing through the deflection magnetic fields are subjected to a diverging effect in a horizontal direction and to a focusing effect in a vertical direction respectively. In the present application, such focusing effect on the electron beams that is larger in the vertical direction than in the horizontal direction due to the diverging effect in the horizontal direction and the focusing effect in the vertical direction is referred to as a negative astigmatism.
When the electron beams come to travel further along with an increase in a deflection angle, because of the self-convergence magnetic field described above, the diverging effect in the horizontal direction and the focusing effect in the vertical direction become distinctive, especially at edge portions of the phosphor screen. Consequently, electron beam spots on the phosphor screen become elongated horizontally such that the major axis is parallel to the horizontal direction, causing a problem of lowering a horizontal resolution. The recent development of flatter panels and larger deflection angle makes the above problem more and more serious.
In general, when the deflecting device has the self-convergence structure, the above-described deformation of the spot shape occurs easily. Thus, in order to display images with high resolution on the phosphor screen, the electron gun has to achieve a smaller spot diameter in the horizontal direction.
Although various factors generally influence the spot diameter in the color display tube device, the spherical aberration of a main lens contributes most to the spot diameter in its relationship with the main lens of the electron gun. In other words, the spot diameter can be reduced with a decrease in the spherical aberration of the main lens of the electron gun. When an incident angle of the electron beam into the main lens is indicated by xcex1, the spot diameter xcex4 is expressed by
xcex4=(Mxc2x7Cspxc2x7xcex13)/2
where M is a lens magnification and Csp is a spherical aberration coefficient. When the focusing effect of the main lens is weakened, the lens magnification and the spherical aberration are reduced. One of the methods for weakening the focusing effect of the main lens is to increase an equivalent diameter of the main lens. In other words, by increasing an effective main lens diameter, it is possible to achieve a smaller spot diameter on the phosphor screen.
JP 2(1990)-18540 B discloses a conventional main lens in an electron gun for a color display tube. As shown in FIG. 11, the main lens is constituted by a focusing electrode 32, a final accelerating electrode 33 and a shielding cup 34 connected to the final accelerating electrode 33. The focusing electrode 32 and the final accelerating electrode 33 are spaced away from each other in a tube axis direction. The focusing electrode 32 and the final accelerating electrode 33 respectively include peripheral electrodes 35 and 36 surrounding three electron beams 8a, 8b and 8c and electrode plates (hereinafter, referred to as xe2x80x9cvertical electrode platesxe2x80x9d) 37 and 38. The vertical electrode plates 37 and 38 are retracted with respect to the opposing end faces of the peripheral electrodes 35 and 36 and arranged so as to allow the electron beams to pass substantially perpendicular thereto.
FIG. 12 shows a front view of the vertical electrode plates 37 and 38. Openings 39a, 39b, 39c in the vertical electrode plate 37 and openings 40a, 40b and 40c in the vertical electrode plate 38 are formed to have a horizontal diameter smaller than their vertical diameter.
The vertical electrode plates 37 and 38, which are arranged inside the respective peripheral electrodes 35 and 36 of the focusing electrode 32 and the final accelerating electrode 33 constituting the main lens, are retracted as described above, thereby allowing a high electric potential of the final accelerating electrode 33 to enter deeply into the focusing electrode 32 and a low electric potential of the focusing electrode 32 to enter deeply into the final accelerating electrode 33. This increases the effective main lens diameter, thereby achieving a smaller spot diameter on the phosphor screen.
Also, the negative astigmatism, in which the lens focusing effect is stronger in the vertical direction than in the horizontal direction owing to the peripheral electrodes 35 and 36 having their diameter larger in the horizontal direction, is eliminated by making an opening diameter in the horizontal direction Rh smaller than that in the vertical direction Rv in the vertical electrode plates 37 and 38 so as to prevent the entrance of the electric potential in the horizontal direction.
However, such an electron gun in the conventional color display tube device has had a limitation in increasing the main lens diameter and adjusting the astigmatism concurrently. In this technique, the vertical electrode plates are retracted in order to increase the main lens diameter. On the other hand, however, the horizontal diameter of the openings of the vertical electrode plates is reduced in order to adjust the astigmatism, resulting in a limitation in increasing the main lens diameter especially in the horizontal direction.
The object of the present invention is to provide a color display tube having a main lens portion structure that can adjust astigmatism easily and achieve a higher horizontal resolution by increasing a main lens diameter in a horizontal direction with a relatively simple configuration.
In order to achieve the object mentioned above, a color display tube of the present invention includes an envelope including a front panel on which a phosphor screen is formed and a funnel, and an in-line electron gun that is provided in a neck portion of the funnel and emits three electron beams. The in-line electron gun has a focusing electrode and a final accelerating electrode that are disposed facing each other in a tube axis direction so as to have a predetermined space therebetween and constitute a main lens, and a shielding cup that has a bottom provided with at least one opening through which the electron beams pass and is connected to a side of the phosphor screen of the final accelerating electrode via the bottom of the cup. A vertical electrode plate having three openings formed in-line through which the three electron beams pass respectively is provided only inside the focusing electrode out of the focusing electrode and the final accelerating electrode. A horizontal electrode plate that is substantially parallel to an in-line plane is formed so as to extend toward the focusing electrode at least one of above and below the opening provided at the bottom of the shielding cup. The horizontal electrode plate is only inside the final accelerating electrode out of the focusing electrode and the final accelerating electrode.
This configuration makes it possible both to increase the main lens diameter in the horizontal direction as desired and to adjust the astigmatism. Thus, even when the horizontally-elongated distortion of the spots becomes distinct at edge portions of the phosphor screen owing to the development of flatter panels and larger deflection angle, the horizontal spot diameter is reduced by the electron gun, thereby alleviating the horizontally-elongated distortion. In this manner, it becomes possible to provide the color display tube device with a high resolution.
It is preferable that the number of the openings formed at the bottom of the shielding cup is three, and the three openings all have a circular shape.
With this configuration, an assembly jig used in assembling the electron gun can have a circular shape, eliminating the need for a complex shape. Thus, it becomes easier to process the assembly jig, and further to assemble the electron gun.
Also, it is preferable that at least one of the openings provided at the bottom of the shielding cup has a non-circular shape.
With this configuration, an astigmatism amount provided to the three electron beams can be adjusted individually. Also, it is possible to restrict a variation of the astigmatism amount by a punching accuracy of the openings alone.
Furthermore, it is preferable that a central opening of the three openings provided in the vertical electrode plate has a diameter in an in-line direction smaller than that in a direction perpendicular to the in-line direction.
With this configuration, it is possible to eliminate the negative astigmatism, in which a lens focusing effect on the electron beam passing through the central opening is larger in the vertical direction than in the horizontal direction owing to a peripheral electrode having its diameter larger in the horizontal direction.
In addition, it is preferable that a height of the horizontal electrode plate in the vicinity of a central beam of the three electron beams is different from that in the vicinity of beams on both sides.
With this configuration, not only can the main lens diameter in the horizontal direction be increased, but also the astigmatism amount provided in the three electron beams can be adjusted individually.